Molar Mass of Carbon Dioxide
PA State Standards:
3.7.10.B
Apply appropriate instruments and apparatus to examine a variety of objects and processes.
3.4.10.A
Explain concepts about the structure and properties of matter.
3.1.10.E
Describe patterns of change in nature, physical and man made systems.
2.5.11.A
Select and use appropriate mathematical concepts and techniques from different areas of
mathematics and apply them to solving non-routine and multi-step problems.
1.2.11.A
Read and understand the central content of informational texts and documents in all academic
areas.
Introduction:
The Ideal Gas Equation (PV = nRT) may be used to determine the molar mass of a gas or
vapor. The pressure, volume, mass, and temperature of the gas must be known. These
are all quantities that can be easily measured in the laboratory. The equation is used to
solve for the moles of the gas. From the number of moles and the mass of the vapor, the
molar mass is determined.
Guiding Question:
Please answer the following question before beginning the lab.
What is the molar mass of oxygen according to the following data?
Data:
Mass of empty bottle
30.16 g
Mass of bottle + O2
32.47 g
Mass O2
2.31 g
Pressure O2
Volume of bottle
Temperature
R
1907 mm Hg
720 mL
296 K
62.4 L · mm Hg · K-1 · mol1
What is the percent error?
Molar Mass of Carbon Dioxide
Revised 6/26/09
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Science in Motion
Equipment/Materials:
1 L graduated cylinder
cap (fitted with valve stem)
dry ice
PETE beverage bottle
Juniata College
pressure gauge
thermometer
tongs
top-loading balance
Safety:
 Goggles should be worn in the lab.
 Use only tongs or cryogenic gloves to handle the dry ice. Do not handle the dry ice
with your bare hands; frostbite can result.
Procedure:
1. Obtain a polyethylene terephthalate (PETE) beverage bottle that has been fitted with
a tire stem. Make sure that the interior and exterior are dry. Weigh the bottle with
the cap and record the mass in the data table.
2.
Place a sample of approximately five grams of dry ice in the soft drink bottle. Shake
the bottle vigorously to sublime the dry ice quickly. When enough of the dry ice has
been converted to gas to fill the bottle with CO2 (leaving no air), put the cap on.
Continue shaking the bottle until all the dry ice has sublimed. Use care in handling
the dry ice; it can cause frostbite.
3.
When all the dry ice has sublimed, there may be frost on the bottom of the bottle;
thaw it under running water, and thoroughly dry the bottle.
4.
Weigh the bottle on the balance, and then check the pressure in the bottle using the
tire pressure gauge. Record the mass and pressure values in the table.
5.
Determine the temperature of the room using the thermometer. Record the
temperature value in the data table.
6.
Fill the bottle with water and pour it into a graduated cylinder to obtain the volume
of the bottle; record the value in the data table.
Data:
Mass of bottle + air
Mass of bottle + CO2
Pressure from gauge
Temperature
Volume of bottle
Molar Mass of Carbon Dioxide
Revised 6/26/09
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Science in Motion
Juniata College
Calculations:
1. Atmospheric pressure in the English system is often measured in psi (pounds per
square inch). Standard pressure is 14.7 psi. The type of pressure gauges we are
using measure pressures above atmospheric pressure. Add standard pressure to the
reading you obtained from the gauge.
2.
Using the combined gas law, convert the experimental volume of the bottle to the
volume at STP. Remember to convert psi to atm.
3.
Using 1.29 g/L as the density of air at STP, calculate the mass of air in the “empty”
bottle.
4.
Determine the mass of the “empty” bottle by subtracting the mass of air from the
mass of the bottle + air.
5.
Determine the mass of CO2 by subtracting the mass of the “empty” bottle from the
mass of the bottle + CO2.
6.
In order to use the ideal gas law, the value of R must be known. Depending upon
which units of measurement are used, this value differs. The gas constant for this
lab can be determined by plugging the STP values for volume (in liters) per mole of
gas, temperature (in Kelvin), and pressure (in atmospheres) into the ideal gas law.
What is the gas constant in
? Show your work.
Molar Mass of Carbon Dioxide
Revised 6/26/09
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Science in Motion
Juniata College
7.
Substitute the appropriate values into the PV = nRT equation, and solve for moles.
8.
Using the mass and the number of moles of carbon dioxide, calculate the molar mass
of carbon dioxide.
9.
Calculate the percent error.
Questions:
1. Before capping the bottle, some of the dry ice should be allowed to sublime. Why
might an incorrect molar mass be obtained if this step is not done?
2.
Why does the volume of the bottle need to be converted to STP before the mass of
air in the bottle can be determined?
3.
If carbon dioxide escapes as the pressure is checked, how will the molar mass value
be affected? Explain your reasoning.
4.
If the graduated cylinder is read incorrectly and the recorded volume is too high,
how will the molar mass value be affected? Explain your reasoning.
Molar Mass of Carbon Dioxide
Revised 6/26/09
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Science in Motion
5.
Juniata College
If there is water on the outside or inside of the bottle when the final mass is recorded,
how will the molar mass value be affected? Explain your reasoning.
Additional References:
This lab, “Gas Experiments with Plastic Soda Bottles”, originally appeared in the
November 1998 issue of the Journal of Chemical Education, p 1405.
Molar Mass of Carbon Dioxide
Revised 6/26/09
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